With global decreases in species diversity, the effects of species diversity on aboveground biomass of forest communities, especially tropical forests which store nearly half of terrestrial carbon, have received much attention. It is commonly assumed that species diversity can promote the accumulation of aboveground biomass in forest communities. However, increasing evidence suggests that this positive effect of species diversity can be influenced or neutralized by structural diversity and various abiotic environmental variables. It is still far from clear whether scale dependent effects of various drivers of aboveground biomass exist. Using structural equation modeling, we examined direct and indirect drivers of aboveground biomass in a 50 ha tropical forest dynamics plot on Barro Colorado Island (BCI), Panama. Our results showed that the effect of species diversity on aboveground biomass was non-significant at the smallest scale and became significantly negative at larger scales. The strength of this negative effect increased with spatial scales. Conversely, structural diversity had a significant positive effect on aboveground biomass and the strength of this effect decreased with scale. Abiotic variables had some relationships with both types of diversity and can indirectly affect aboveground biomass. These results suggest that positive relationships between species diversity and aboveground biomass in conventional studies may be caused by structural diversity. The relationship between